Who is responsible for a delay in the scrum backlog
Let us assume, based on the scrum velocity, the scrum team has got 16 story points and the team has completed only 12 story points. In the consecutive iterations, the team's commitments have come down to 12 story points. Can the stake holders ask the scrum master about the decrease in the team's commitment? If the project continues with more sprints than planned, the seller organization is going to loose their planned profit percentage if it is a fixed price project otherwise the buyer organization. Anybody in the organization is accountable for the same?
The team should not be making commitments to an amount of work or velocity. The team's commitment should be the Sprint Goal. Using estimates and forecasts may help teams understand if a Sprint Goal is reasonably likely to be achieved.
There is a potential conversation here, though. The Scrum Team has a cost per Sprint. If the value of the Sprint Goal is not worth the cost of the Scrum Team for that Sprint and this continues for an extended period, then there may be conversations on how best to make the Sprint Goals more valuable. Maximizing value delivery is the accountability of the Product Owner, and the Scrum Master may be in a position to help facilitate improvements that the team can make to maximize value delivery.
Anybody in the organization is accountable for the same?
The Product Owner is accountable for value. If the Developers are committing to points, and not to a valuable Sprint Goal evidenced by at least one Done and immediately usable increment of work, find out why.
How much revenue does a story point generate for your company? Now for the bigger question, why would you have that information?
A story point is a guess made by someone at a specific point in time based upon the information that they had at that time. If new information is uncovered during the work, the original estimate is no longer valid.
As @Thomas points out, there is a fixed cost per Sprint for the Scrum Team. There is a potential profit value that can be gained by the organization based upon the contract that was signed. However, all of that was based upon what the stakeholders needed at the time the contract was signed. You do not mention when that was or how long the expected development time is, but I am going to assume it is more than 1 month based upon your comments. So, has anyone asked the stakeholders if their needs have changed since the contract was signed? Have the stakeholders been giving feedback on all of the increments delivered by the Scrum Team to help determine next steps and changes so that what is ultimately delivered meets the needs that they have at the time it is delivered?
I'll answer @Ian's question for you below. You seem to want a fixed time, fixed cost, fixed solution. That does not need any agility to create. It is standard Microsoft Project Plan so do it that way. However, if the stakeholders want the ability to adapt the work that is incrementally delivered so that what is ultimately given to them satisfies the needs they have at that time it is delivered, redo the contract. Do a contract that is "pay as you go, stop when you have what you want" and bill them a fixed amount per Sprint that will allow the Scrum Team to be paid a good salary and the company can still make a profit.